Facebook has an ugly little secret, a number disclosed nowhere in its voluminous filings to become a public company and now only vaguely addressed by corporate officials.

Facebook has an ugly little secret, a number disclosed nowhere in its voluminous filings to become a public company and now only vaguely addressed by corporate officials.

An estimated 5.6 million Facebook clients - about 3.5% of its US users - are children who the company says are banned from the site.

Facebook and many other web sites bar people under age 13 because the Children's Online Privacy Protection Act (COPPA) requires web sites to give special treatment to children 12 or younger. The law aims to stop marketers prying personal information from children or using their data to advertise to them. Sites must get parental permission before allowing children to enter, and must take steps to protect privacy.

Facebook declines to acknowledge that many of its efforts to block children are not working.

The issue has taken on new relevance as the Federal Trade Commission finalizes rules to further restrict companies and Web sites that target youths or are geared to young audiences.

Facebook, the world's leading social media company with 955 million users, has said that the law does not apply to it because it explicitly restricts use of its site to people aged 13 and older.

Facebook has made some progress in identifying preteens and excluding them from the site. A June Consumer Reports study showed that Facebook eliminates as many as 800,000 users under age 13 in a year through its tiered screening process, which the company declines to describe.

The study still estimates 5.6 million children are on Facebook, a figure that experts say includes many who create accounts with help from their parents.

The Consumer Reports data comes from a January 2012 survey of 2,002 adults with home Internet. Participants were chosen by TNS, a research firm. The margin of error is plus or minus 2%age points.

"It's not surprising to us to see 12-year-olds sneaking onto Facebook," said FTC Chairman Jon Leibowitz, saying the situation was "particularly complicated" if parents helped them. "Is it troubling? In some ways it is. Is it a story in black and white? Not really."

A Reuters test of Facebook's signup process shows that a child could bypass the site's screening features with relative ease. The site effectively blocked a fictitious sign-up from an underage prospective user. But after an hour's wait, the site accepted a sign-up using the same name, email, password and birthday but citing a different birth year.

Facebook declined to discuss the data or describe its efforts to outlaw children. Spokesman Frederic Wolens said in an email that Facebook is "committed to improving protections for all young people online".

Larry Magid, who serves on Facebook's advisory board and co-directs the Internet group Connect Safely, said he and others studied the issue for a year and found no way to tell if children were lying online.

"The only solution that I am aware of is to access some sort of national ID or school records," he said. "There are good reasons that we don't do this. ... I'm sure this is really easy to do in totalitarian regimes."

Senator Richard Blumenthal, an outspoken privacy advocate whose youngest child is 18, said children's vulnerability to potential sexual predators and susceptibility to advertising were reasons to keep the 12-and under set off most web sites. "Our children were not on Facebook at that age, and they would not be now," he said.

When gullible preteens or "tweens" go online they often reveal sensitive data, said Kathryn Montgomery, who teaches at American University and was an early advocate of the 1998 COPPA Tlaw.

"What we hoped to do with these kinds of rules is to get companies to act responsibly toward kids. It's not easy to do," said Montgomery.

Facebook now boasts 158 million US users, according to May figures from the data firm comScore. If the site more effectively banned children, it could stand to lose about 3.5% of its US market.

Ironically, one reason it's easy to game Facebook's screening process is the law passed to protect children. COPPA bars companies from saving most data on children. The FTC has said it would look skeptically on companies saving childrens' names or email addresses even if the data simply helped them prevent children logging onto their sites.

Children who aren't savvy enough to game Facebook's system often get parental help, according to a 2011 study headed by Danah Boyd, a senior researcher at Microsoft Research. She found that 55% of parents of 12-year-olds said that their child was on Facebook and that 76% of those had helped the child gain access.

"Many recent reports have highlighted just how difficult it is to enforce age restrictions on the Internet, especially when parents want their children to access online content and services," said Facebook's Wolens.

On Facebook, children are exposed to advertising for sugary, high-fat foods, the kind increasingly pulled from children's television shows.

"We found lots of food products on Facebook being advertised, including many which are targeted to children," said Jennifer Harris, director of marketing initiatives at Yale University's Rudd Center for Food Policy and Obesity.

Krave Cereal Is One-Third SugarOne is Kellogg's new Krave cereal, a product which is roughly one-third sugar. With advertisements featuring an animated, pudgy Krave Krusader, it now counts 456,000 "likes" on Facebook.

Kellogg's said it did not intend to market Krave to tweens and complied with an industry initiative to not market high-fat, high-sugar products to children. "Krave follows Facebook's policy that all fans must be 13 or older," the company said in a statement.

Dr. Victor Strasburger, chief of the division of Adolescent Medicine, University of New Mexico Department of Pediatrics, said the Krave Krusader ads are part of what he called "unethical" appeals by sugary cereal makers. Nearly 20% of US children aged 6-17 are obese, according to a 2011 government report.

Child advocates say that even if Facebook is not appealing directly to children, the company needs to realize that ads aimed at teenaged users will also attract tweens, who imitate older peers.

"I don't think Facebook deliberately goes out and gets kids at the moment," said Jeff Chester of the Center for Digital Democracy. "I think when they target teens the way they do, they know that they'll pull in a lot of younger kids."